A noise cancellation technique of cancelling external noise at, e.g., a speaker device or headphones so that a user's ears can hear only musical sound has been typically in widespread use. According to such a noise cancellation technique, the external noise is detected by a microphone, and then, a noise cancellation signal with a phase opposite to that of the detected noise signal is generated. Subsequently, the noise cancellation signal is output from, e.g., the speaker device to cancel the external noise.
Meanwhile, a full digital speaker device configured so that a digital signal can be directly input to a speaker has been recently developed. This full digital speaker device can directly transfer the digital signal to the speaker, and therefore, digital/analog conversion is no longer required. Thus, high-quality sound can be realized regardless of performance of a digital/analog converter.
However, when the above-described noise cancellation technique is applied to the full digital speaker device, a delay of about 0.5 msec to 3 msec is, due to a delay caused by an arithmetic circuit of a digital filter portion provided in the full digital speaker device, caused after input of a noise signal until output of sound.
For this reason, when an attempt is made to perform signal processing for the input noise signal to remove noise as in the typical noise cancellation technique, a noise-processed signal delay corresponding to the signal processing is also caused, and a delay in response to actual noise is caused. Thus, there is a problem that effective noise reduction cannot be performed.
In order to prevent such a delay, a device has been typically proposed, which includes a speaker unit having a single diaphragm and two voice coils configured to drive the diaphragm and which is configured such that a musical sound signal is input to one of the voice coils and a noise cancellation signal based on a noise signal detected by a noise detection microphone is input to the other voice coil, for example (see Patent Literature 1).